310 THE SALIVARY GLANDS. 



The salivary alveoli are bounded externally by a basement membrane, 

 which, however, appears in some cases to be incomplete, owing to the 

 flattened cells which compose it being branched and stellate instead of 

 epithelioid. Moreover these cells send processes inwards which form a 

 sustentacular network amongst the salivary cells * 



A delicate intra-alveolar reticulum is described by various obseiTers, enclosing- 

 in its meshes the alveolar cells. It is at present micertain Avhether the appear- 

 ance is to be referred to the sustentacular network of cell-processes above 

 mentioned, or to the network of intercellular passages shown in fig. 239, "or to an 

 independent cause. 



In sections of the submaxillary gland there is to be seen here and there in the 

 alveoli a peculiar lialf-moon-sliai^ed granular mass, staining deeply with carmine, 

 and lying between the salivary-cells and the enclosing basement membrane 

 (fig. 2;]8, c). This body, which was fii-st noticed by Gianuzzi. and has been 

 regarded as of considerable importance, we are inclined to look upon, -wdth 

 Pfliiger, as most probably due to a post-mortem change consequent on the 

 presence of mucus within the more central cells — these becommg swollen hy 

 imbibition, and compressing the marginal cells, which contain no mucus, against 

 the basement membrane. In accordance with this view it may be stated that the 

 .semilunar body is not found in those animals (rabbits) in which the salivary-cells 

 contain no mucus, nor in the glands of other animals (dogs) the cells of which 

 have been caused to discharge their contained mucus by ii-ritation of the nerves 

 proceeding to the gland (Heidenhain). 



The smallest ducts of the gland, those into which the alveoli open, 

 are composed only of a basement membrane and a simple layer of 

 flattened epitheliimi. After a short course the character of the 

 epithelium changes rather abruptly, the cells becoming large and 

 columnar, smallest towards the lumen of the tube, and each containing- 

 a roundish nucleus near the centre (fig. 233, a). The part of the cell 

 nearest the lumen of the duct is homogeneous or granular in character, 

 whereas the part nearest the basement membrane appears finely striated 

 longitudinally. In the ducts of the parotid gland this striated appear- 

 ance is said to be absent. 



Pfliiger states that the striation is due to the presence of excessively fine varicose 

 fibrils, and that these are directly continuous with those forming the axis-cylinders 

 of nerves, which he describes as penetrating the basement membrane. 



Tire free ends of the cells, which project into the lumen, present a mosaic 

 apijearance when seen from the surface. 



The larger ducts acquire a coating of fibrous 

 Fig. 239. n^■y^^ elastic tissue outside the basement membrane, 



and, except in those of the sul)lingual gland, a 

 few plain muscular fibre-cells are also to be found : 

 the columnar epithelium becomes at the same time 

 gradually shorter and shorter, until in the main 

 -, '■ ducts it is cubical or even tesselated. 



Fig. 239. — An Alveolus of a Compound Racemose Gland, 



INJECTED FROM THE EXCRETORY DuCT. HiGHLY MAaNIFIED. 



Hardly anything but the dark injecting fluid is sho^^^l ; the 

 alveolar cells and nuclei are only faintly indicated ; those of the 

 duct are not represented at all. The injection is seen filling 

 the central cavity of the alveolus, and passing from this in fine 

 chani.els (represented by black reticulating Hues), between and 

 around the cells (after Saviotti). 



According to the observations of Pfliiger and Ewald, the central cavities of the 

 * Boll, Arch. f. milu-. Anat. v. 



